Triallists and leagues that let you play them.

Does anyone know the real world general rules for triallists? I know that in the Scottish leagues teams can play triallists because you often see A Triallist having scored for a team in the Sunday papers, but are the rules in FM representative of the real world for every nation? I'm playing in Iceland and there seems to be no rule that I can't play triallists in league games, but it just seems silly that you'd be allowed to, especially with it being the Icelandic Premier League. I feel like I'd be cheating and I have limited myself to playing triallists in reserve matches and the League Cup.

Is there a FIFA rule or is it down to the FA of a nation as to what the rules are? How accurate is FM on the whole?